England's biggest classroom union issued a threat yesterday to strike unless the next government cuts teachers' workload to a 35-hour week.

The annual conference of National Union of Teachers (NUT), which represents most of the profession, voted unanimously for a motion calling for a ballot for industrial action as early as this summer if ministers did not agree to their demands.

The threat comes days after the Tories called for schools to open on Saturdays to offer extra lessons to the poorest children. Teachers argued that they work an average of 18.7 hours of unpaid overtime every week, and that their workload has grown worse. The motion, proposed by teachers in east London, warned that "reducing teacher workload is vital to improve both teachers' working conditions and children's learning conditions".

A spokesman from the Department for Children, Schools and Families said the government had already made "massive strides" to reduce teachers' workload. Ministers had given teachers extra planning time and limited the number of hours they could be asked to cover for absent colleagues, he said.

Tomorrow teachers will vote over whether to walk out if ministers cut school budgets and freeze their pay.

Also at the NUT conference in Liverpool, Mick Brookes, leader of the National Association of Head Teachers, gave a rallying call for teachers to vote to boycott national tests for 10- and 11-year-olds. The NUT and the NAHT are balloting heads and their deputies over whether to "frustrate the administration" of the tests next month. Brookes said: "If [the government] aren't going to change the system, then we will change it for them. I am proud we are standing up for what matters."

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